Field
Inventive concepts described herein relate to semiconductor memory devices, and more particularly, to memory systems capable of deciding a read level in high speed, and read methods thereof.
Description of Conventional Art
Semiconductor memory devices may be volatile or nonvolatile. The volatile semiconductor memory devices may perform read and write operations at high speeds, while contents stored therein may be lost at power-off. The nonvolatile semiconductor memory devices may retain contents stored therein even at power-off. The nonvolatile semiconductor memory devices may be used to store contents, which must be retained regardless of whether they are powered.
A flash memory device may be a typical nonvolatile semiconductor memory device. The flash memory device may be widely used as a voice and image data storing medium of information appliances such as a computer, a cellular phone, a PDA, a digital camera, a camcorder, a voice recorder, an MP3 player, a handheld PC, a game machine, a facsimile, a scanner, a printer, and the like.
With an increase in a need for a large-capacity memory device, a multi-level cell (MLC) or multi-bit memory device storing multi-bit data per cell is becoming more common. However, memory cells in an MLC memory device must have threshold voltages corresponding to four or more discriminable data states in a limited voltage window. For improvement of the data integrity, levels of read voltages for discriminating the data states must be adjusted to have optimal values.